Submarine canyon plastic accumulation is driven by turbidity currents, simulation study shows
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Plastic pollution is an ever-pressing concern for the health of our ocean ecosystems and their inhabitants, with estimates of over 10 million metric tons of plastic litter entering the marine realm each year. While evocative images often highlight surface plastics with organisms such as turtles and sea lions caught in fishing nets and plastic bags, the issue spans much deeper. In fact, during the exploration of the deepest part of the ocean (Mariana Trench) in 2019, a plastic bag was found at nearly 11,000 meters below sea level.
While smaller particulate plastics may sink through the water column to the seabed, larger plastic litter is proposed to accumulate through large gravity-driven mass movements of sediment-laden water, known as turbidity currents. These turbulent flows can travel at speeds up to 20 meters per second, eroding the seafloor and expanding submarine canyons while also transporting vast quantities of sediment, nutrients and pollutants to the deep sea.
New research published in Geophysical Research Letters has simulated the strength of correlation between turbidity currents and plastic litter accumulation in these underwater canyons.
To do so, Dr. Yupin Yang of China's Southern University of Science and Technology and colleagues used a combination of bathymetric data (underwater seafloor topography and elevation) and video footage from manned submersible dives of the Qiongdongnan region of the South China Sea between 2018 and 2020, alongside computational fluid dynamics models.
Combining these datasets, the model tested a range of hydraulic parameters to predict sediment dispersal patterns over various seafloor topographies across simulations running for 5.5 hours. This included setting a canyon inlet size of 100 meters where the turbidity current began and maintaining set sediment sizes, particle densities, sediment concentrations, grain size distributions, flow thicknesses and flow velocities.
They identified submarine canyon morphology as the primary driver of turbidity current flow and deposition patterns, leading to sites of plastic litter accumulation. Specifically, concave scours up to 30 meters deep encouraged deposition of a turbidity current's entrained debris, with 88% of the plastic litter confined to four such features in the study area.
Dr. Yang and the team state that shear velocity, referencing the stresses at the base of the turbulent flow, is the key driver exerting control over the deposition of the turbidity current's sediment and plastic debris (here, predominantly plastic bags). Consistently, they found reduced shear velocity correlated with turbidite deposition.
This research is important as plastics behave differently to sediments in the turbidity flows, where their density does not automatically correlate with known deposition patterns. Instead, the buoyancy of plastics can lead to them having greater mobility, traveling larger distances across the seafloor. Additionally, more energetic, and therefore erosive, turbidity currents at a later point can capture these previously-deposited plastics and carry them further still into the depths of the marine realm for many years to come.
More information: Yuping Yang et al, How Does Plastic Litter Accumulate in Submarine Canyons?, Geophysical Research Letters (2024). DOI: 10.1029/2024GL110767
Journal information: Geophysical Research Letters
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